Post-installed reinforcement bars in structural connections

Transcription

1 Post-installed reinforcement bars in structural connections Jorge Gramaxo Business Unit Anchors Hilti Corporation Chile 21 November,

2 Agenda Applications and different methods Internal stresses in the connections Qualification of an adhesive mortar Designing and detailing the connection Conclusions 2

3 Post-installed bars are often required to correct jobsite errors but can also be a favored method Misplaced or missing bars Due to incorrect positioning - or - bars movement when pouring and vibrating the concrete Consciously planning - Contractor preferred method (e.g. to avoid jobsite errors) - Easier/only possibility due to construction process (e.g. slurry walls) - Extension of existing building (e.g. height extensions) - Strengthening / retrofitting for seismic loading 3

4 Post-installed bars grants flexibility required in new constructions or renovation works (1/2) Connection of new slabs Balcony extensions Close openings / slab enlargement Staircase connections 4

5 Post-installed bars grants flexibility required in new constructions or renovation works (2/2) New columns / Columns extensions Bridge enlargement Beam connections Wall extension 5

6 Some of the methods for post-installing rebars are limited to new construction Mechanical couplers Pre-cast rebar sets Perforated formwork Very attractive method for pre-fabricated slabs Risk of misplacement Localized reduction of the bars cross-section Limited to new projects Relatively easy to install and straighten the bars Risk of misplacement Typically pricy solution Limited to smaller bars Limited to new projects Positioning of the bars mostly guaranteed Holes in formwork neutralizes reutilization Leakage of cement reduces concrete quality in the connection Limited to new projects 6

7 Using high-level adhesives combines safety with jobsite flexibility and efficiency Chiseling, Welding Cementitious grouts Adhesive mortars Load transferred directly to the existing bars Risk of creating concrete cracks along the bars High dependency of the quality of the welding Welding can t be done in unfavorable weather Low cost of grout per milliliter Can only be installed downwards or inclined Dubious control over the mixture done on jobsite Typical large drilling diameter vs bar size Mixture quality ensured by dispenser units Small sized diameter and high bond strength Drilling required Higher cost of mortar per milliliter 7

8 Post-installed rebar application are studied and designed in a very different way than anchors MATERIALS: Chemical mortar + reinforcement bar (aka rebar) + APPLICATION: Connecting new concrete to existing concrete new existing This clearly referred to as a postinstalled rebar connection Anchoring connection that uses a rebar in lieu of a threaded bar Adhesive mortars are used but this is the only similarity to anchoring 8

9 Agenda Applications and different methods Internal stresses in the connections Qualification of an adhesive mortar Designing and detailing the connection Conclusions 9

10 A rebar under tension develop in the concrete compression and resulting tension stresses 10

11 To avoid direct tension on the concrete, tension loads are usually transferred by lap splices For a single rebar, the tension forces are transferred via compression struts into the concrete, the concrete would be taking up tension! Splices are created exactly to ensure that tension is transferred from one bar to the other bar releasing the concrete from tension stress. 11

12 Shear also develop tension in the concrete and stirrups aid to transfer stress to the supports Real stresses Strut and Tie module However, not only the stirrups transfer the shear to the supports 12

13 Even in concrete sections in pure shear stress, longitudinal bars are always under tension For a simply supported element, the bottom longitudinal bars will always be in tension, even at the support; in this case due to shear Load on longitudinal bars: moment & shear xxxxvvxxxx 13

14 When no splice is present, there must be always a strut to avoid concrete failures in tension The bar is in tension yet the surrounding concrete is in compression which releases the concrete from tension stress. Compression (cord, strut) Interface Tension (cord) Tension (tie) In the case of post-installed bars there must be a qualification 14

15 The load transfer of a post-installed rebar must be assessed to be said equivalent to cast-in Cast-in rebar Post-installed rebar Rebar Concrete Rebar Mortar Concrete 1. mechanical interlock between ribs and the concrete 1. mechanical interlock between ribs and mortar layer 2. adhesion and micro interlock from mortar into the concrete Adhesion and installation of a post-installed rebar must be checked 15

16 Agenda Applications and different methods Internal stresses in the connections Qualification of an adhesive mortar Designing and detailing the connection Conclusions 16

17 Rebar resistance Regulation landscape EU and US guide the assess of mortars for post-installed rebars Both EU (since 2006) and US (since 2013) frameworks guide the assessment of adhesive mortars for post-installed rebar applications. US (UBC / IBC) EU (EOTA) Qualification ( testing ) ICC-ES AC308 ACI EOTA TR023 Technical data ( approval ) ICC-ES Evaluation Service Report: ESR European Technical Approval: ETA Design method ACI 318 Chapter 12 Eurocode 2 EN

18 The testing is extremely comprehensive and check reliability, installation and performance The tests (13 sets!) are conducted to establish suitable in terms of: - Bond strength for low and high concrete strength - Bond transfer behavior ensured even if splitting controls - Sensitivity to cleaning, extreme service temperatures, installation orientation, alkalinity/acidity exposure - Embedments of 60d, aptitude of the system (mortar, installation gear) - Corrosion protection of the rebar - Seismic tension load behavior (US only) The full system performance is crucial in the qualification, not only adhesive bond strength determined in optimum conditions 18

19 One key assessments is to identify if the mortar is able to transfer the load as a cast-in (1/2) Bond/splitting behavior is crucial to allow and harmony of load transfer between the post-installed and the cast-in bars in a splice. Reference test : Cast-in bar spliced with another cast-in bar F F University of Stuttgart SPIETH, OZBOLT, ELIGEHAUSEN, APPL Numerical and Experimental Analysis of Post-Installed Rebars Spliced with Cast-in- Place Rebars. International Symposium on Connections between Steel and Concrete. 19

20 steel stress [Mpa] One key assessments is to identify if the mortar is able to transfer the load as a cast-in (2/2) Bond/splitting behavior is crucial to allow and harmony of load transfer between the post-installed and the cast-in bars in a splice. HIT-RE 500 used in splice Epoxy resin X used in splice F F F F cast-in-place Hilti HIT-RE embedment depth [mm] F=20kN F=40kN F=60kN F=100kN Fmax=116kN Not all epoxy have the same behavior! Too high stiffness hints to an unexpected splitting 20

21 Some mortars have a huge sensitivity to the drilling method, even more than to cleaning Even if it seems a jobsite topic, this should be covered during the design and specification of the post-installed rebar solution. Anticipating jobsite mistakes, it s suggested to specify a mortar system approved to hammer and diamond drilling 21

22 Hole cleaning impact of adhesive mortars is also of highest relevance in rebar applications Traditional essential process after drilling Cleaning is not required using a mortar approved for drilling with the Hilti Hollow Drill Bits (automatic dust removal during drilling) 22

23 Finally, it must be guaranteed that there will be mortar throughout the embedded rebar It s quite anticipated that for embedments above 250mm (10in) during a traditional injection air pockets will remain in the hole. OK: Load fully transferred, theory meets practice NOT OK: current situation not sufficient to transfer load The are solutions to ensure zero air pockets as it is the case of Hilti Piston Plug 23

24 Agenda Applications and different methods Internal stresses in the connections Qualification of an adhesive mortar Designing and detailing the connection Conclusions 24

25 Embedment for static loading and seismic ordinary / intermediary ductility structures ACI318 Chapter 12 expression to calculate development length: l bd 40 a f 3 y t e s d Min. embedment for tension = 12in b f ' c cb K tr d b Simplification of the expression for PI rebars (max c/d = 2.5): l bd a K tr 3 f y cx s s s c = min(cx, cy, s/2) d 40 f ' c Max (c/d) = 2.5 c cy d 1.0 Bar #7 (0.875in) 0. 8 Bar < #7 (0.875in) s s Assuming normal weight concrete 1.0 Assuming non-coated rebars No influence of reinforcement t 1.0 e Do not experience top-bar effects 25

26 Embedment for seismic loading in special ductility reinforced concrete moment frames ACI318 Chapter 21 expression to calculate development length: l l d d f f y f ' y f ' c c d d b b Depth of the concrete cast in one lift beneath the bar 12 in Depth of the concrete cast in one lift beneath the bar > 12 in Comparison to the standard Chapter 12 calculation For c/d = 2.5 (max value) Bar size #7 l bd 1 33 f y f ' c d b Depth of the concrete cast in one lift beneath the bar 12 in Increase factor: 1.25 Depth of the concrete cast in one lift beneath the bar > 12 in

27 Emdedment (in) Emdedment (in) Development lengths = cast-in (ACI 318 Chapter 12) concrete 4000psi, steel Gr.60, static, ordinary/intermediate seismic Straight bars Confinement* (in) *edge distance or ½ rebar spacing #3 #4 #5 #6 #7 #8 #9 # concrete +25% 6000 concrete -20% Spliced bars Confinement* (in) *edge distance or ½ rebar spacing 27

28 Designing per any concrete code, splitting and bond failure are the failure modes to precluded Generically... Rebar close to an edge Rebar away from the edge 28

29 Bond that can be developed Bond that can be developed For straight bar connections, embedment is limited by concrete splitting until bond governs Very close edges or spacing No edges, no near bars or Mortar bond limit Limited by mortar bond Concrete* limit Limited by concrete* Cast-in Post-inst. Splitting limits the resistance Cast-in Post-inst. Bond limits the resistance * bond (mechanical interlock) between rebar ribs and the concrete HIT Rebar method bridge the 2 opposite cases 29

30 Utilizable bond strength f bd [ N/mm 2 ] Due to extensive 20 years of internal testing Hilti is able to identify the bond/splitting behavior for cases above Extension for post-installed bars with large cover (unique for Hilti) Bond strength of used adhesive (deppending on mortar and conditions) 3.5 ACI concrete «bond» limit c d /f 30

31 Emdedment (in) Drill bit TE-YX 21 (also available 36 ) Emdedment (in) Drill bit TE-YX 36 Embedment lengths all possible methods concrete 4000psi, steel Gr.60, continuous row of straight bars ACI 318 Chapter 12 (cast-in) ACI 318 Chapter 12 extension (Hilti) Bar #6 (f19) Bar #8 (f25) % -46% % -38% Spacing (in) 2500 concrete +25% 6000 concrete -20% Spacing (in) 31

32 Post-installing a rebar is not only the right selection of adhesive but it s truly a system Compared to the traditional anchor embedments, the longer embedments installation highlight the importance of installation Anchor design ~10d/~15d (ACI 318 Appendix D) Increased installation importance Equal to cast-in ~35d/45d (ACI 318 Chapter 12) Surface preparation: - Roughening the interface Drilling: - Different drilling methods Cleaning the hole: - Removing the dust Injection: - Avoiding air pockets The specification of a post-installed rebar connection should indicate clearly surface preparation and installation gear in lieu of just mortar 32

33 Agenda Applications and different methods Internal stresses in the connections Qualification of an adhesive mortar Designing and detailing the connection Conclusions 33

34 Main takeaways Among all methods for post-installing rebars, adhesive mortars best combine safety and flexibility for both design and installation. Post-installed rebars can ensure a truly monolithic connection and as a result the bars will be only under tension, as regular cast-in bars. In the case of properly qualified adhesive mortars, the design of postinstalled rebars can be done exactly as a cast-in. A proper specification should include the bars diameter and spacing, the drilling depth, the adhesive mortar yet also installation notes. For the specific case of Hilti solutions, due to the extensive testing, embedments of straight bar connection can be significantly reduced. 34

35 Thank you for your attention! Jorge Gramaxo Business Unit Anchors Hilti Corporation Chile 21 November,